Sound. Sound, the stimulus for hearing, is made up of a series of pressures, usually of air, that can be represented as waves. Sound waves have three characteristics—amplitude, frequency, and purity—each of which is related to a psychological experience. Greater wave amplitudes are related to greater loudness; wave frequency is related to pitch; and wave purity is related to timbre.

The hearing system. The outer ear, the pinna, collects sound waves and funnels them through the auditory canal to the eardrum (which separates the outer and middle ears) and causes it to vibrate. (See Figure .) The middle ear contains the malleus (hammer), incus (anvil), and stapes (stirrup), which move and transmit the sound to the oval window, which separates the middle ear from the inner ear. Beyond the oval window is the inner ear, whose main structure is the cochlea, a snail‐like structure that has a membrane, the basilar membrane, stretched along its length. When the stapes vibrates against the oval window, the fluid in the cochlea moves and causes the basilar membrane to vibrate. The receptors for hearing, the hair cells, lie in the basilar membrane and convert the vibrations into neural impulses. The neural impulses, in turn, move along the auditory nerve to the lower brain stem and then ascend to the auditory part of the thalamus and on to the auditory cortex in the temporal lobe. Input from each ear is received on both sides of the brain.

Figure 1
The Ear